This invention relates to a bipolar separator and, in particular, to a bipolar separator for use in carbonate fuel cells.
The bipolar separator used in carbonate fuel cells is required to provide current transmission, reactant gas separation and load pressure distribution in a high temperature corrosive environment. The separator is exposed to a reducing gas mixture of H2, N2, H2O, CO and CO2, on the anode side and to an oxidizing gas mixture of O2, N2, H2O, and CO2 on the cathode side. Moreover, it must withstand fuel cell operating temperatures near 650xc2x0 C. in the presence of a liquid alkaline carbonate electrolyte.
A typical bipolar plate of this type comprises a thin flat metal plate member having first and second opposing surfaces which form active areas and which are compatible with oxidant and fuel gases, respectively. First and second opposing ends of these areas are folded upward and toward each other to form first and second pockets. Third and fourth opposing ends of the areas are folded downward and toward each other to form third and fourth pockets. The first and second pockets support a fuel cell matrix on the first surface side of the plate member, while the third and fourth pockets support a fuel cell matrix on the second surface side of the plate member. These pockets, thus, form wet-seals for the carbonate fuel cells in which the bipolar separator is employed.
A significant concern in designing a bipolar separator is fabricating the plate member so that the wet seals and edges of the plate member are kept corrosion free for realizing long term stability. The wet-seals are wetted by the molten carbonate electrolyte in the presence of both reducing and oxidizing atmospheres. This greatly accelerates the corrosion attack. However, because the wet-seals are not in the electrical pathway, high electrical conduction is not required. On the other hand, the active areas of the plate member provide the electrical conduction. These areas are thus required to conduct generated electrons and, therefore, need to be highly conductive in order to minimize any internal ohmic dissipation.
Currently, one technique for fabricating the bipolar separator is to form the separator from a Ni-coated or clad stainless steel sheet. This type of fabrication is desirable because pure Ni remains metallic in the low oxygen partial pressure fuel environment of the carbonate fuel cell, while stainless steel is somewhat more stable in the oxidizing cathode environment of such cell.
However, when such a sheet is used to fabricate the bipolar plate, an additional corrosion protection coating is required for the aforementioned wet-seals of the sheet to alleviate the more aggressive hot corrosion which takes place at these areas. This additional coating is required to be uniform in thickness and nearly dense. At the carbonate fuel cell operating temperature, interdiffusion between the coating and the base metal forms a highly corrosion resistant protective layer.
To date, the wet-seal protective coating is usually formed from Al or Al/Fe (see, e.g., Japanese Patent 09025822). This coating may be applied by thermal spray (see, e.g., Japanese Patents 09025822, 07295276), high velocity oxy-fuel flame spray (HVOF) (see, e.g., U.S. Pat. No. 5,698,337), Al painting, ion vapor deposition, or molten Al dip-coating (see, e.g., Japanese Patent 07230175). Currently, the thermal spray techniques, such as conventional thermal spray and HVOF, are widely utilized.
When using the thermal spray techniques to apply the Al coating, considerations relating to coating adhesion and substrate warpage, require that the coating be applied after the bipolar separator has been formed. This necessitates handling and masking of the large thin stainless steel sheet during the coating process. The need to handle and mask the sheet adds significantly to the bipolar separator manufacturing costs.
Furthermore, extensive control must be exercised over the coating process to meet the quality required in fuel cell use. A satisfactory coating can only be achieved by following costly quality control procedures, including pre-surface preparation, coating thickness and roughness inspections, visual defect and coverage inspection and periodic metallographic examination.
Additionally, even using these procedures, the resultant coating still forms a thick loose layer of aluminum oxide on top of the interdiffusion zone during fuel cell operation. This causes undesired flaky debris at the exposed surfaces. To avoid the flaky oxide layer and achieve a defect-free interdiffusion protective zone, a high-temperature diffusion heat treatment prior to the fuel cell application is required. This treatment introduces undesired thermal distortion to the bipolar plate.
As above-described, the pockets forming the wet seals of the bipolar separator are situated at the ends of the metal plate and extend either downward or upward and then toward each other. If Ni-coated stainless steel is used for the bipolar separator, one set of pockets will have Ni on their exterior surfaces. This Ni has to be removed before coating the pockets with Al, since applying the Al coating to the Ni provides insufficient corrosion protection in the fuel cell environment.
As for the active-area formed by the opposing first and second surfaces of the bipolar plate, a high-Cr stainless steel is favored for corrosion protection on the cathode side surface. However, excessive Cr content may lead to poorly electrical conductive corrosion products and additional electrolyte loss.
It is therefore an object of the present invention to provide a bipolar separator which avoids the above-mentioned disadvantages.
It is a further object of the present invention to provide a bipolar separator which can be easily and cost-effectively fabricated.
In accordance with the principles of the present invention, the above and other objectives are realized in a bipolar separator in which the separator comprises a plate member having opposing first and second surfaces compatible with fuel gas and oxidant gas, respectively, and first and second pocket members and third and fourth pocket members which are separately formed from the plate member. In accord with the invention, the first and second pocket members are joined to opposing first and second ends of the plate member, respectively, by welds, so that the first and second members extend outward and upward of the plate member and then toward each other. Also, in accord with the invention, the third and fourth pocket members are joined to opposing third and fourth ends of the plate member, respectively, by welds, so that the third and fourth pocket members extend outward and downward of the plate member and then toward each other.
By forming all the pocket members separately from the plate member, the pocket members can be fabricated from a material which itself is highly resistant to corrosion in the fuel cell environment. More particularly, in accord with the invention, a stainless steel material provided with a corrosion resistant cladding is employed. Preferably, the cladding is made of aluminum.
Additionally, the separately formed plate member can now be fabricated from a material having characteristics desired for the first and second surfaces of the plate which form the active areas of the separator, i.e., high electrical conductivity, high corrosion resistance and low electrolyte loss. In accord with the invention, a stainless steel material is used. This material may have a nickel cladding.
In a further aspect of the invention, the welds employed to attach or affix the pocket members to the plate member are themselves formed to provide corrosion resistance. Preferably, Al is introduced into each weld to achieve the desired corrosion resistance.